SRU: backport Python 3.6.12, 3.7.9 and 3.8.6 to bionic

[Matthias Klose]

This is a follow-up to LP: #1835737, backporting Python 3.6.12, 3.7.9 and Python 3.8.6 to bionic.

The 3.7 and 3.8 packages are not used by default (Python 3.6 is the default), so there is no regression potential for the packages.

The python3.8-doc package isn't built, because a newer sphinx version that the one in bionic is required.

The updates for 3.6 are checked with a test rebuild.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Neal McBurnett (nealmcb)]

There seems to be at least one security issue with the current version of Python 3.8 in Bionic, which is still 3.8.0.

See: What’s New In Python 3.8 — Python 3.8.6 documentation
 https://docs.python.org/release/3.8.6/whatsnew/3.8.html

I.e.: Notable changes in Python 3.8.1
 Due to significant security concerns, the reuse_address parameter of
 asyncio.loop.create_datagram_endpoint() is no longer supported. This is because of the behavior of
 the socket option SO_REUSEADDR in UDP. For more details, see the documentation for loop.create_

So thanks for putting this together, and I hope it is accepted soon.